1. Field of the Invention
The present invention relates to an organic functional element, particularly, an organic semiconductor element, an organic thin film transistor element (hereinafter, a thin film transistor may be referred to as a TFT), an organic electroluminescence element (hereinafter, an electroluminescence may be referred to as an EL), and a method for producing the same.
2. Description of the Related Art
In recent years, a flat display has been used in many fields and places, and importance thereof has increasingly enhanced with advancement of informatization. Currently, a representative of a flat display is a liquid crystal display (also referred to as LCD) and, as a flat display based on different display principle from that of LCD, organic EL, inorganic EL, a plasma display panel (also referred to as PDP), a light emitting diode display (also referred to as LED), a vacuum fluorescent display (also referred to as VFD), and a field emission display (also referred to FED) have been also actively developed. All of these new flat displays are called self-emitting types, are considerably different from LCD in the following points, and have excellent characteristics which are not possessed by LCD.
LCD is called as a light receiving type, and a liquid crystal does not emit the light by itself, works as a so-called shutter which permeates and shields the light from the light source, and constitutes a display. For this reason, LCD requires a light source, and generally requires back light. To the contrary, a self-emitting type does not require another light source since an apparatus itself emits the light. In the light receiving type such as LCD, a back light is always turned on irrespective of a state of display information, and consumes approximately the same electric power as that of the full display state. To the contrary, the self-emitting type consumes an electric power only at a place requiring turning on depending on display information, it has in principle an advantage that electric power consumption is small as compared with the light receiving-type display.
Similarly, in the LCD, in order to obtain the dark state by shielding the light of a back light source, it is difficult to completely exclude light leakage even if the quantity of light is small, while in the self-emitting type, no emitting state is just the dark state, it is easy to obtain an ideal dark state, and the self-emitting type is predominantly prevails also in a contrast.
In addition, since the LCD utilizes polarization control by virtue of birefringence of a liquid crystal, so-called viewing angle dependency in which the display state is greatly changed depending on a direction of observation is great, but there is little of this problem in the self-emitting type.
Further, since the LCD utilizes an orientation change derived from dielectric anisotropy of a liquid crystal which is an organic elastic substance, a response time to an electric signal is in principle 1 ms or longer. To the contrary, in technique of the aforementioned self-emitting type which is being developed, since transition of a so-called carrier such as electron/hole, electron release, plasma discharge or the like is utilized, a response time is in an order of ns, which is a dramatically high rate as compared with a liquid crystal, and there is no problem of animation afterimage derived from slowness of response of LCD.
Among them, particularly, an organic EL has been actively studied. An organic EL is also called as an OEL (Organic EL) or an organic light emitting diode (OLED).
An OEL element and an OLED element have a structure in which a layer (EL layer) containing an organic compound is disposed between one pair of electrodes, i.e. an anode and a cathode, which is based on a laminated structure of “anode/hole injection layer/light emitting layer/cathode” of Tan et al (for example, see Japanese Patent No. 1,526,026). Tan et al. use a low-molecular material while Henry et al. use a high-molecular material (for example, see Japanese Patent No. 3,239,991).
In addition, an efficiency is improved by using a hole injection layer or an electron injection layer, and an emitting color is controlled by doping a light emitting layer with a fluorescent dye or the like. Also, in an organic EL, high luminance light emission can be obtained at relatively low voltage driving like as 10 V or less, application as an illuminating device in place of a fluorescent lamp having a problem of use of mercury is expected.
FIG. 10 is a schematic view showing a basic cross-sectional structure of a conventional organic EL element 101. An organic EL element 101 has a basic structure in which an organic material layer containing at least a light emitting layer (EL layer) 104 is disposed between an anode 103 and a cathode 105, and by applying the electric field between both electrodes to flow a current through the EL layer, the light is whereby emitted. The light emitting layer 104 may be a multilayered structure provided with an auxiliary layer such as a hole injection layer 106 or an electron injection layer 107, if necessary.
Usually, an organic EL element 101 is produced by forming a transparent electrode as an anode 103 on a translucent substrate 102 such as a glass substrate and a plastic substrate due to a work function of a transparent electrode such as ITO with respect to an energy level of an EL layer, and thereafter, forming an EL layer which is a light emitting layer 104, and a cathode 105 which is a counter electrode in this order. In the organic EL element 101 having such a construction, light emission 108 can be observed from a transparent electrode (anode 103) side.
Conventionally, an organic EL element has utilized only fluorescent emission when returned from the singlet excitation to the ground state, but in recent study, effective utilization of phosphorescent emission when returned from the triplet excitation state to the ground state has been enabled and the efficiency thereof improved.
A transparent electrode can be prepared by sputtering or vacuum-depositing a transparent electrically conductive film of ITO or IZO on a transparent substrate, separately from formation of an EL layer thereafter.
As a forming method of the EL layer, generally, when a low-molecular material is used as a material for the EL layer, a vacuum evaporation method using a mask is used. In the case of a high-molecular material, the material is formulated into a solution, and an ink jet method, a spin-coating method, a printing method, a transferring method or the like is used. In recent years, a low-molecular material which can be coated has been also reported.
Among them, in a mask vacuum evaporation method of a low-molecular material, from restriction that scale up of a vacuum device and a depositing mask is difficult, therefore, there is a problem that meeting scale up and production of many pieces using a large substrate are difficult. This means that there is no problem at a scale of around experimental production at a development stage, but market competitiveness is weak in terms of a tact and a cost at a regular production stage.
On the other hand, in a high-molecular material, and a low-molecular material which can be coated, since a film can be formed by a wet process such as an ink jet method, a printing method, a casting method, an alternate adsorbing method, a spin-coating method, a dipping method or the like, there is little problem of meeting a large substrate as aforementioned, and as a method of forming an organic EL element, a coating process is promising. For example, by dissolving PPV (poly(p-phenylenevinylene)), which is a high-molecular organic EL material described in Japanese Patent No. 3,239,991, in an organic solvent, and spin-coating the solution on a transparent electrode, an EL layer can be formed.
Finally, for example, a film of a low work function metal such as Al, Ag or the like is made by vacuum vapor deposition to obtain a cathode. However, in the aforementioned production method, since a film of a cathode is made by vapor deposition, a vacuum device requiring a laborious work becomes necessary only in a step of forming a cathode, and a production tact is stopped due to vacuuming, therefore, there is a problem that characteristic of an organic EL material which can be formulated into a film by coating, cannot be sufficiently utilized.
Another characteristic of an organic EL composed of a coated film is flexibility. When an element is constituted using a flexible substrate such as a resin, a plastic or the like, a soft, so-called flexible element can be produced and, also in this case, a cathode has been problematic in the prior art. Even when a substrate and an organic material layer are made to have a flexible structure, if a metal thin film formed by vapor deposition as conventional is used as an electrode, breaking cannot be avoided when bended.
As utilization of Ga and a Ga alloy, which are a liquid metal, as an electrode in a broad sense, there is the invention of use in an electric connecting means (see Japanese Patent Application Laid-Open (JP-A) No. 5-74,503). This pays an attention to easiness in bonding and peeling easiness of a liquid metal, and utilizes the liquid metal in a contact point of a connector pin or the like, but utilization as an electrode for flowing electricity through an organic material layer or acting the electric field on the organic material layer in order to exhibit function of an organic functional element is not described therein.
In an organic functional element, it is important to inject more charges, particularly electrons, in an organic material layer. A substance having a low work function has better electron injection effect. In this point, an alkali metal and an alkaline-earth metal are suitable. In the conventional organic EL element produced by vapor deposition or the like, it is general to laminate an alkali metal or an alkaline-earth metal and other metal. In addition, use of an alloy of an alkali metal or an alkaline-earth metal and other metal in an electron injection electrode of an organic EL element has been proposed (for example, see JP-A No. Hei. 9-320,763, JP-A No. Hei. 10-12,381 and JP-A No. Hei. 11-329,746).
However, in organic EL elements described in JP-A No. Hei. 9-320,763, JP-A No. Hei. 10-12,381 and JP-A No. Hei. 11-329,746, since an alkali metal and an alkaline-earth metal are unstable for having strong oxidizing property and flammability in the air, it is difficult to handle them and, conventionally, a film was made only by vapor deposition under vacuum.
In the technique described in JP-A No. Hei. 9-320,763, JP-A NO. Hei. 10-12,381 and JP-A No. Hei. 11-329,746, for example, an alloy region containing an alkali metal or an alkaline-earth metal is formed near a light emitting layer by vapor-codepositing various metals each of which is used as an independent vapor deposition source, and an electrode is formed by vacuum vapor deposition. Although other techniques use an alloy of an alkali metal or an alkaline-earth metal and other metal, an electrode is formed by a vapor deposition method or a sputtering method using an alloy as a target material.
The aforementioned problems are not limited to an organic EL element. There is the similar problem in all organic functional elements composed of an organic material layer and an electrode.